The invention pertains to a method for operating an internal combustion engine with at least one working piston with a variable compression ratio.
A device by means of which the closing point of the intake valve of an internal combustion engine can be varied, so that the compression ratio can be varied as a function of the operating state of the internal combustion engine, is known from DE 4,108,454 C2. A knock sensor, which is mounted on the engine housing in the known internal combustion engine, is provided to detect when the compression ratio is too high. When the knock sensor responds, thus signaling that the compression ratio is too high, an adjusting mechanism changes the closing time of the intake valve. When the operating state changes again in such way that it is possible for a higher compression to be used, provision is made in the known internal combustion engine for detecting the temperature of the wall of the combustion chamber. When this temperature is below a predetermined value, the closing time of the intake valve can be changed, as a result of which the compression ratio is increased again. The disadvantage of this internal combustion engine is that the open-loop or closed-loop control system functions relatively slowly, especially when the compression ratio is to be increased, because the temperature of the wall of the combustion chamber changes relatively slowly, i.e., the temperature detection process is relatively slow.
A method for operating a four-cycle internal combustion engine is known from DE 198-04,988 C1. The compression ratio is varied by changing the opening and closing time of the intake and/or outlet valve. So that the compression ratio can be adjusted to the optimum value, various sensors are provided to monitor the combustion process. For example, an ion current sensor is provided inside the combustion chamber. In addition, a knock sensor, designed as a structure-borne sound sensor, is attached to the engine housing. It is found that, because of the large number of sensors, especially in the case of multi-cylinder engines, it is necessary to process or evaluate large amounts of sensor data. The electronic circuitry, i.e., the control mechanism, must therefore be relatively complicated.
It is therefore the task of the invention to provide a method for operating an internal combustion engine with at least one working piston, which method does not suffer from the disadvantages mentioned above.
This task is accomplished by a method for operating an internal combustion engine with at least one working piston, which method has the features stated in claim 1. This internal combustion engine has a variable compression ratio, which can be changed as a function of the operating state of the internal combustion engine, a knock sensor being used to detect when the compression ratio is too high, so that the compression ratio can then be lowered below the knock limit. According to the invention, the process is characterized in that, when the knock limit is reached or exceeded, the compression ratio is reduced by a first, predetermined value, which is a function of the operating state, over a first, predetermined time period, and in that the compression ratio is then increased by a second, predetermined value over a second, predetermined period of time until it reaches or exceeds the knock limit. The decrease in the compression by the first value and/or the increase in the compression by the second value preferably occurs in a continuously falling manner. A graphic depiction on a two-axis orthogonal coordinate system, in which the compression ratio is plotted versus time, yields a graph of the compression ratio in the form of a sawtooth-like curve. The compression ratio is lowered and then raised again as a function of time by specific, predetermined values, which depend on the operating state of the internal combustion engine. The compression ratio therefore xe2x80x9coscillatesxe2x80x9d continuously around the knock limit or stays below it. Because the compression ratio therefore is continuously being raised and lowered in the area of the knock limit, the maximum possible compression ratio is always obtained, no matter what the operating state of the internal combustion engine. It is advantageous in this method that only the measurement values of the knock sensor need to be evaluated, which means that complicated combustion monitoring sensors inside the combustion chamber are not required. In addition, the effort which must be make to implement the closed-loop or open-loop control of the system is simplified, because only the signal of the knock sensor must be processed.
It is provided in accordance with a further elaboration that the first value and/or the first time period is determined as a function of the load demand on the internal combustion engine. The first value and/or the first time period will be different, depending on the load range in which the internal combustion engine is operating, so that, overall, the sawtooth course of the compression ratio curve will be composed of steeper or flatter sections, when seen on the coordinate system.
If, therefore, it can be recognized under dynamic operating conditions, that is, under conditions of changing loads, that the compression ratio must be decreased quickly as a result of a sudden high load demand, the first value and/or the first time period and/or the second value and/or the second time period must be changed in order to meet the demand for a rapid decrease in compression. It can therefore be provided that the first value is increased and/or the first time period decreased in order to achieve a rapid decrease in compression, which will then be followed by an increase in compression. If, for example, the knock sensor responds again within, especially at the end of, the first time period, the first value by which the compression is decreased is increased, and it keeps being increased until the knock sensor no longer responds within the first time period. Then the first value is set back to its baseline value, which depends on the operating state.
If, under dynamic operating conditions, the compression ratio is to be increased quickly as a result of a sudden drop in the load demand, the second value by which the compression is increased within the second time period may possibly not be sufficient to meet the demand for a rapid increase in compression. If, therefore, the knock sensor does not respond within, especially at the end of, the second time period, the compression increase is repeated at a higher second value until the knock sensor starts responding again as the compress ratio increases. This leads again to a decrease in the second value of the compression increase to the baseline value associated with the operating state of the internal combustion engine.
In another elaboration of the invention, it is provided that operating parameters of the internal combustion engine and/or of a motor vehicle driven by the internal combustion engine are taken into consideration in the determination of the first value, of the first time period, of the second value, and/or of the second time period. Such parameters might include, for example, the rpm""s of the engine; the amount of fuel injected; the position of the pedal sensor; the temperature of the air, coolant, and/or oil; the temperatures of the exhaust gas and/or catalyst; the ambient air pressure; the charging pressure; the flow rate of the intake air; the driving speed; and/or the ignition timing.
The invention is especially applicable with advantage to direct-injection engines, especially to spark-ignition engines. The invention is also applicable especially to internal combustion engines, preferably engines for motor vehicles, which are equipped with NOx storage catalysts.